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Originally published in Science Express on 5 August 2004
Science 27 August 2004:
Vol. 305. no. 5688, pp. 1277 - 1280
DOI: 10.1126/science.1101304
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Reports

Evidence for Deep Magma Injection Beneath Lake Tahoe, Nevada-California

Kenneth D. Smith,1* David von Seggern,1 Geoffrey Blewitt,1,2 Leiph Preston,1 John G. Anderson,1 Brian P. Wernicke,3 James L. Davis4

A deep earthquake swarm in late 2003 at Lake Tahoe, California (Richter magnitude < 2.2; depth of 29 to 33 kilometers), was coeval with a transient displacement of 6 millimeters horizontally outward from the swarm and 8 millimeters upward measured at global positioning system station Slide Mountain (SLID) 18 kilometers to the northeast. During the first 23 days of the swarm, hypocentral depths migrated at a rate of 2.4 millimeters per second up-dip along a 40-square-kilometer structure striking north 30° west and dipping 50° to the northeast. SLID's transient velocity of 20 millimeters per year implies a lower bound of 200 nanostrains per year (parts per billion per year) on local strain rates, an order of magnitude greater than the 1996 to 2003 regional rate. The geodetic displacement is too large to be explained by the elastic strain from the cumulative seismic moment of the sequence, suggesting an aseismic forcing mechanism. Aspects of the swarm and SLID displacements are consistent with lower-crustal magma injection under Lake Tahoe.

1 Nevada Seismological Laboratory, University of Nevada, Reno, NV 89557, USA.
2 Nevada Bureau of Mines and Geology, University of Nevada, Reno, NV 89557, USA.
3 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 92215, USA.
4 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA.

* To whom correspondence should be addressed. E-mail: ken{at}seismo.unr.edu

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